1. Field of the Invention
The present invention relates to electrostatographic imaging and recording apparatus, and particularly to assemblies in these apparatus for fixing toner to the substrates.
2. Description of Background and Other Information
Generally in electrostatographic reproduction, the original to be copied is rendered in the form of a latent electrostatic image on a photosensitive member. This latent image is made visible by the application of electrically charged toner.
The toner thusly forming the image is transferred to a substrate, such as paper or transparent film, and fixed or fused to the substrate. The fusing of toner to substrate can be effected by applying heat, preferably at a temperature of about 90xc2x0 C.-200xc2x0 C.; pressure may be employed in conjunction with the heat.
A system or assembly for providing the requisite heat and pressure customarily includes a fuser member and a support member. The heat energy employed in the fusing process generally is transmitted to toner on the substrate by the fuser member. Specifically, the fuser member is heated; to transfer heat energy to toner situated on a surface of the substrate, the fuser member contacts this toner, and correspondingly also can contact this surface of the substrate itself. The support member contacts an opposing surface of the substrate. Accordingly, the substrate can be situated between the fuser and support members, so that these members can act together on the substrate to provide the requisite pressure in the fusing process.
During the fusing process toner can be offset from the substrate to the fuser member. Toner thusly transferred to the fuser member in turn may be passed on to other members in the electrostatographic apparatus, or to subsequent substrates subjected to fusing.
Toner on the fusing member therefore can interfere with the operation of the electrostatographic apparatus and with the quality of the ultimate product of the electrostatographic process. This offset toner is accordingly regarded as contamination of the fuser member, and preventing or at least minimizing this contamination is a desirable objective.
U.S. Pat. No. 5,217,837 discloses a toner fusing system which utilizes internal heating, and includes a fuser member having a 30-65 micrometer thick fusing surface layer over a 1-3 millimeter thermally conductive HTV silicone elastomer layer. U.S. Pat. Nos. 5,017,432 and 5,332,641 also disclose toner fusing systems characterized by, inter alia, internal heating and fuser members with fluoroelastomer surface fusing layers.
Toner fusing systems using external heating are also known. U.S. Pat. Nos. 4,372,246, 4,905,050, 4,984,027, and 5,247,336 all disclose external heating for the toner fusing function. Of these, the latter three teach a configuration with the fuser roller situated between, and in contact with, two heating rollers.
A factor in achieving sufficient fusing quality is providing sufficient heat transfer from the fusing surface layer of the fuser member to the substrate toner. In the prior art it has been understood that this heat transfer is improved by increasing the thermal conductivity of the fusing surface layer of the fuser member, and it has also been understood that the thermal conductivity of this layer is increased by increasing its content of heat conducting filler. Yet additionally in the prior art, it has been understood that providing the fusing surface layer with particular minimum amounts of the filler is necessary to obtain sufficient thermal conductivity, sufficient heat transfer, and sufficient fusing quality.
However, heat conducting filler particles in the fusing surface layer provide high energy sites for removing toner from the substrate. Therefore, increasing the amount of heat conducting filler content in the fusing surface layer, by providing more reactive sites for the toner, increases toner offset and therefore also increases contamination of the fuser member.
It has not been known or suggested in the prior art, but with the toner fusing system of the present invention, it has been discovered that fusing quality is maintained even where the quantity of heat conducting filler in the fusing surface layer is reduced, or in fact this filler is absent. Accordingly, the system of the present invention has the advantage of allowing for reducing the heat conductive filler content of the fusing surface layer, thereby lessening toner contamination, while still providing effective fusing of toner to substrate.
It has further been discovered that in a toner fusing system which utilizes external heating as its primary heat source, and which also utilizes a fuser member with one or more cushion layers and a fluoroelastomer fusing surface layer, the thickness of the fluoroelastomer fusing surface layer is critical to operation of the system. Specifically, this feature has been found to be critical to minimizing contamination of this layer, and also to providing it with sufficient strength, wearability, and resistance to delamination.
In this regard, it has surprisingly been discovered that keeping the fluoroelastomer fusing surface layer within a particular maximum thickness significantly lessens the indicated contamination during operation of the system. It has yet additionally been discovered, as a matter of particular surprise, that this lessening of contamination is not affected by the thermal conductivity of the layer as a whole, by the amount of heat conducting particles therein, by the identity of these particles, or by the degree of thermal conductivity characterizing the particular type of heat conducting particles employed. Still further, it has surprisingly been discovered that keeping this layer at or above a particular minimum thickness provides this surface fusing layer with sufficient strength and wearability, and especially resistance to delamination.
The assembly, or system, of the invention includes a fuser member. The fuser member comprises a fuser base as well as at least one cushion layer and a fusing surface layer. The at least one cushion layer and the fusing surface layer successively overlay or reside on the fuser base, with the at least one cushion layer being interposed between the fuser base and the fusing surface layer.
The fusing surface layer serves to contact toner on the substrate, and can further contact the substrate surface on which the toner resides. The fusing surface layer comprises at least one polyfluorocarbon elastomer, or fluoroelastomer, and has a thickness of from 38 microns, or 1.5 thousandths of an inch, to 178 microns, or 7 thousandths of an inch.
The invention also includes one or more external heating members, and optionally yet additionally includes one or more internal heating members. The heating member or members are for heating the fuser base and the layers residing thereon, or at least for heating the fusing surface layer. The heated fusing surface layer in turn heats the toner, thereby providing the necessary heat energy for the fusing process.
The at least one external heating member is the primary heat source for the toner fusing system of the invention. Where at least one internal heating member is also present, it acts as a secondary heat source for the toner fusing system of the invention.
The invention preferably also comprises a support member for cooperating with the fuser member. Specifically, with a substrate located between the fuser member and support member, they cooperate to exert pressure on the substrate during the fusing process; the fuser and support members define a nip that the substrate passes through, thereby providing appropriate pressure for the fusing process.